In electrostatographic apparatus such as copiers and printers, automatic adjustment of process control parameters is used to produce images having well regulated darkness or optical density. Copier and printer process control strategies typically involve measuring the transmissive or reflective optical density of a toner image on an exposed and developed area (called a "test patch") of an image receiver. Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage, especially for transmission density, is that density is approximately proportional to the thickness of the marking material layer, over a substantial range.
Typically, toned process control test patches are formed on the photoconductor in interframe regions of the photoconductor, i.e., between image frame areas. An "on-board" densitometer measures the test patch density, either on the photoconductor or after transfer of the patches to another support member. From these measurements, the machine microprocessor can determine adjustments to the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
A transmission type of densitometer is particularly well suited to transmissive supports. In this type, a light source projects light, visible or infrared, through an object onto a photodetector such as a photodiode. In a copier/printer, the photoconductor passes between the light source and the photodetector. When the photoconductor has toner on the surface, the amount of light reaching the photodetector is decreased, causing the output of the densitometer to change. Based on this output, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer as described in U.S. Pat. No. 4,553,033 to Hubble, III et al, uses reflected light flux rather than transmitted light flux to determine density, and is particularly suited to opaque reflective supports.
Typically, the copier/printer logic and control unit (LCU) is burdened with tasks that involve managing the data collection procedure and processing large amounts of raw density data presented to the LCU. To obtain a single net toner density measurement, the untoned base density must be measured, and then subtracted from the toned density measurement. The LCU must provide the proper timing signals for these readings, receive the readings, provide the necessary storage, and perform the subtraction. The untoned base density readings must be updated from time to time so that changes in the base or in the densitometer itself are properly accounted for. The base material may wear or otherwise change its optical transmission or reflection characteristics. The densitometer light emitter typically degrades with age, and the emitter and detector typically become contaminated, for example, with toner dust and paper dust.
U.S. Pat. No. 5,122,835 to Rushing et al discloses a process for measuring untoned areas during cycle-up and between image frames during imaging cycles, storing the readings, and subtracting the stored readings from subsequent readings. In this scheme, the densitometer provides individual density readings of defined spots on an image belt. The LCU coordinates the timing of these readings, stores readings, and performs the arithmetic operations to correct the raw density readings. These functions are in addition to the routine LCU functions of controlling the operation of the various printer workstations.
For process control purposes, individual density measurements are typically averaged, filtered, or otherwise mathematically processed in real-time. Such mathematical processing is often for the purpose of reducing the effects of various noise sources. This enables more precise operation of the process control, and better regulation of image quality. Noise may arise, for example, from toning nonuniformity or electrical noise coupled into the densitometer from other parts of the machine. Ordinary averaging of N noisy and uncorrelated readings can reduce the standard deviation, of the averages, by a factor of about N.sup.1/2, compared to single readings.
An LCU must be provided with enough speed, memory, input/output capability, and software to provide these functions simultaneously with real-time control of the printer subsystems. U.S. Pat. No. 5,075,725 to Rushing et al, for example, discloses an electrophotographic process control utilizing averaging of multiple densitometer readings. Four readings are taken in each process control patch and averaged. Further real-time calculations determine adjustments to be applied to the electrophotographic process.
For test and diagnostic purposes, copier/printers typically provide special modes of operation to evaluate photoconductor uniformity, wear, electrical characteristics, and defects. Some evaluations may be based on surface potential readings from a non-contact voltmeter, and other evaluations based on densitometer readings. Each sensor collects multiple readings, from which statistics are computed such as mean (or average), median, mode, maximum, minimum, variance, standard deviation, root-mean-square, number of readings out-of-limit, etc., and then typically displayed for evaluation. Such statistics may characterize the entire circumference of the belt or drum, or only specific areas such as process control patches. U.S. Pat. No. 5,903,796 to Budnik et al evaluates photoconductors by this statistical approach. In the Budnik et al disclosure, densitometer readings are taken with a spacing of approximately 1.5 mm. Statistics are computed and used to determine acceptability of the photoconductor belt. The processing power of the LCU must be sufficient for these statistical tasks, along with the routine machine control.
Another disclosure by Budnik et al, in U.S. Pat. No. 5,963,761, collects noisy densitometer readings in the vicinity of the seam of an endless belt. The noisy readings are statistically processed to identify the precise location of the seam. Precise seam location is essential for proper timing of the image processing steps.